The present invention relates to automatic machines for grinding and bevelling the glasses of spectacles.
Conventional machines for grinding and bevelling ophthalmic glasses comprise a U-shaped carriage slidably and pivotally mounted on a fixed horizontal shaft, a second horizontal shaft mounted to be rotatable perpendicularly between the branches of the U-shaped carriage, formed by two parts between which the glass to be worked on is clamped, and a third horizontal rotary shaft carrying the various grinding wheels for trimming and bevelling.
When a blank of an optical glass having a thick edge has been cut out in the shape of the frame, the edge of the glass has a cylindrical shape and a bevelling operation must be carried out by guiding the glass so as to form upon its edge a V-sectioned bevel located between the edges of its periphery, this bevel being intended to be engaged in the groove of the bezel of the frame.
Producing such a bevel with a manual control is extremely difficult; recourse has been had to the "free" bevel system in which the edge of the blank is brought into a V-sectioned groove of a grinding wheel and the shaft carrying the glass is allowed to be free to move in translation during the grinding operation.
However, this system does not give a fully satisfactory result.
FR-A-2,456,304 discloses a device for measuring the distance between a given point of the desired contour of an ophthalmic glass and the plane tangent to the pole of the convex surface of this glass.
This device comprises a unit having two tracers, one of which cooperates with the glass and the other with the template mounted on the carriage and movable in a direction parallel to the axis of the template and glass, one of the tracers being in contact with the surface of the glass.
As the glass rotates, the tracer in contact with its surface provides an indication by shifting an index in front of a graduated index.
In this device, following only a single point of a glass for a given angle of the latter requires effecting a correcting calculation owing to the fact that the grinding point moves in a different way depending on the diameter of the grinding wheel and the non-circular shapes of the frames.